A fuel cell system has hitherto been known in which a raw liquid fuel composed of a mixture of methanol with water is injected into a fuel cell evaporator (evaporation chamber) through a raw liquid raw fuel gas injection apparatus to evaporate the raw liquid fuel to thereby produce a raw fuel gas, the resulting raw fuel gas is reformed in a reformer and carbon monoxide contained therein is removed to prepare a raw fuel gas which is a hydrogen enriched gas, and the hydrogen-enriched raw fuel gas is supplied to the fuel cell to generate electricity. Meanwhile, in the case where the fuel cell system constructed as described above is utilized under the conditions that change in the load is extremely large, e.g., in the case of the fuel cell system carried on a fuel cell electric vehicle, if the raw liquid fuel is sharply injected within the fuel evaporator in order to meet the requirement of increasing the operating power, all of the raw liquid fuel cannot be evaporated, sometimes causing residence of the raw liquid fuel (hereinafter referred to as “liquid residence”) in the fuel evaporator. Similarly, the liquid residence easily occurs if the fuel evaporator is not sufficiently heated due to the lacking of the heat value used for in evaporation, for example, at the time of starting the fuel cell system.
When the liquid residence is generated, the liquid residence, which sustained within the fuel evaporator, is evaporated even if the injection of the raw liquid fuel is stopped, generating the raw fuel gas. This unduly results in changing the response of the fuel evaporator for the worse. In the case where the raw liquid fuel is made of a mixture, among the resulting liquid residence, the components is evaporated in the order of easiness of the evaporation and, thus, there causes unevenness in the gas compositions of the raw fuel gas. This sometimes causes the situation where the reformer does not exhibit its performance sufficiently or the situation where carbon dioxide cannot be sufficiently removed, decreasing the performance of the fuel cell.
In light of such a situation, for the purpose of attaining good response of the fuel evaporator in order to effectively prevent the generation of the liquid residence and, at the same time, for the purpose of quickly warming up the fuel evaporator, our Japanese Patent Application No. 11-125366 (not disclosed) suggests, a fuel evaporator 100, as shown in FIG. 38. This fuel evaporator 100 is composed of a body 110 of the fuel evaporator and a superheating portion 150 residing at the downstream of the body 110 of the fuel evaporator, and a raw fuel injection apparatus 140 provided on the upper portion of the body 10.
Into this fuel evaporator 100, is supplied a combustion gas HG (high temperature thermal medium) obtained by catalytically combusting a hydrogen-containing off gas, which is generated in the fuel cell (not shown), in a catalytic combustor (not shown) as a heat source. The combustion gas HG enters from an inlet 112in, and is passed through the inside of a plurality of U-shaped tubes 112 for thermal medium (referred to as thermal medium tubes) provided in a evaporation chamber 111 within the body 110 of the fuel evaporator to reach an outlet 112out. Subsequently, the combustion gas HG is passed through a combustion gas passage 113 provided on the lower portion of the body 110 of the fuel evaporator, and introduced into the superheating portion 150 provided downstream of the body 110 of the fuel evaporator. The raw liquid fuel FL composed of a mixture of methanol with water is injected from the raw liquid fuel injector 140 in the state of mist, is heated on the thermal medium tubes 112 and is evaporated to be the raw fuel gas FG. The raw fuel gas FG is passed through the interior of evaporation tube 151 provided within the superheating portion 150 to be superheated and then introduced into a reformer (not shown) residing at the downstream of the superheating portion 150.
In this fuel evaporator 100, the lower surface 111b of the evaporation chamber 111 in the body 110 of the fuel evaporator also serves as the upper surface 113t of the combustion gas passage 113. Consequently, since heat is also supplied from the lower surface 111b of the evaporation chamber 111, the generation of the liquid residence is prevented and, even if the liquid residence occurs, the liquid can be quickly evaporated. Accordingly, the fuel evaporator 110 is of good response. Also, the warming up of the fuel evaporator 110 can be conducted in a quick manner.
However, the combustion gas HG, which is a heat source of the conventional fuel evaporator 100 is changed depending upon the operation conditions of the fuel cell and, thus, it is required that a required amount of the raw liquid fuel FL should be evaporated using heat of combusting hydrogen and then is supplied to the reactor. However, there is a problem that the situations of the evaporation in the evaporation chamber 111 (e.g., generation of liquid residence) and the temperature of the raw fuel gas FG are changed by various factors such as the change in the heating value supplied (change in the operation conditions), heat mass of the fuel evaporator itself, and change in atmospheric temperature.
In the case where the fuel cell system is carried on an fuel cell/electric automobile, it is required for the fuel evaporator that the raw liquid fuel is quickly evaporated at the time of starting the system or of sharply changing the load, i.e., the raw fuel gas is obtained with much better response. Furthermore, it is desired for driving the reformer under good conditions that the raw fuel gas is supplied at an appropriate temperature without unevenness of the temperature. In addition, if the raw fuel gas having an appropriate temperature range is obtained at the time of heavy load state, the conventional fuel evaporator has a problem that the temperature of the raw fuel gas under middle or low load conditions becomes unduly high.